I made a crystal radio, which worked, and when I used a 30 feet long antenna which I strung up in my house, I was able to faintly hear two stations, which were in the frequency range of 1000-1100. I also tried using a 100 foot long antenna, and I was able to tune in really clearly to the football game, which was at the 680 frequency. So, I thought in the beginning that if I used a longer antenna I would pick up more stations, but now I'm starting to think that if you use a longer antenna, you will pick up lower frequency stations. Is that correct?

Matthew - You are correct in your understanding that higher frequencies will use shorter antenna lengths to achieve resonance. At 580 KHz, the wavelength (λ) is about 1446 feet. At low frequencies, full wavelength antennas of 1/4 mile in length are not practical. Alternatively, wavelengths at various nodes, λ/2, λ/4, λ/8,... λ/64, are acceptable, although not optimum. Your experience of using the 100' length was close to the λ/16 length node. You might find that by trimming the length to 90', you may see some improvement for the 680KHz station, as that would be the λ/16 length.

The crystal set described in the Science Buddies experiment is very basic. The tuning coil, with taps, relies on stray or parasitic capacitance to provide a resonant circuit. You may find that using a variable capacitor across the coil (something in the area of 0-1000 picofarads) may give you improved selectivity.